In a steering system of a motor vehicle which is configured as an electromechanical power steering system, an auxiliary torque and/or an additional steering angle are/is additionally coupled into the steering shaft by an electric drive in a manner which is dependent on the steering angle and the steering torque which the driver exerts on the steering wheel. The apparatus of the generic type serves for this purpose, which apparatus has a gear mechanism which makes it possible to transmit an auxiliary torque which is produced by the electric drive motor to the steering shaft. In preferred embodiments, the gearwheel on the steering shaft is configured as a worm gear to this end, which worm gear meshes with a worm on the output of the drive motor.
The steering torque is determined by means of a torque sensor, to be precise specifically by way of measurement of the relative rotational angle between a steering wheel-side input shaft and a steering gear-side output shaft which are connected to one another in a rotationally elastic manner via a torsion rod (torsion bar) with a defined torsional stiffness. The relative rotational angle and the steering angle, that is to say the absolute angular position of the steering lock, are determined by means of a sensor arrangement which comprises a sensor element which is connected fixedly to a section of the steering shaft so as to rotate with it, for example a magnet arrangement which acts as a transmitter. The angular position of the sensor element is detected by way of an electronic sensing element which is stationary relative to the rotatable steering shaft, forms an assembly, detects, for example, a magnetic field of the sensor element which is dependent on the rotational angle, and outputs it as an electric measurement signal. The measurement signal is input into a regulator which actuates the electric drive of the electromechanical power steering system and thus defines the auxiliary torque which is to be introduced.
It is known in the prior art to arrange the sensor arrangement for angle measurement in an apparatus of the generic type for introducing an auxiliary torque, to be precise, in particular, the sensor element which is connected to the steering shaft. DE 10 2009 028 828 A1, for example, discloses configuring a fitting face on the steering shaft, on which fitting face a sensor element is shrink-fit thermally in order to form an interference fit. However, the fitting face necessarily requires installation space which is available only to a limited extent in an apparatus for introducing an auxiliary torque; in addition, the production of a fitting face is disadvantageously associated with an additional machining step. Moreover, sensor elements with fastening elements made from plastic can be fastened permanently and securely by means of an interference fit only to a limited extent. DE 10 2007 022 282 A1 describes a further possibility for fastening, namely adhesively bonding the sensor element fixedly on the steering shaft by means of a UV-curable adhesive. Here, however, installation space likewise has to be available, and the adhesive bonding is costly in terms of work and time. Finally, DE 10 2011 118 642 A1 has disclosed, instead of a fitting face, first of all attaching a plastic part on the steering shaft, for example by way of press joining, connecting in a positively locking manner or injection molding, to which plastic part a plastic element of the sensor element is then connected in an integrally joined manner. In the case of this procedure, a relatively large amount of installation space is likewise needed. Moreover, the production and machining complexity for the provision and attachment of the additional plastic element on the steering shaft is likewise relatively high.
In view of the above-described problems in the prior art, it is an object of the present invention to improve an apparatus of the type mentioned at the outset, in such a way that the attachment of a sensor element can take place with less complexity and less installation space as far as possible is used.